1. Field of the Invention
The present invention relates to a pneumatic tire, and more particularly to a heavy duty tire used for trucks or buses, having a tread portion having both block-keynote pattern and so-called unidirectional pattern in which inclined grooves extend from the inner side of the tire width direction to the outside at an angle, especially at more than 45.degree., to a plane passing the tire rotational axis (hereinafter referred to as "high angle uni-directional block pattern").
2. Description of the Prior Art
In general, a pneumatic tire has a variety of grooves which are different in direction, shape and so on at the tread portion in view of appearance, drainage and so on, to define a particular tread pattern.
Also, it is known that in tires for passenger cars it is advantageous in obtaining high drainage performance to form inclined grooves which extend from the inner side of the tire width direction to the outside at an angle, especially at more than 45.degree., to a plane passing the tire rotational axis, that is a high angle uni-directional pattern.
It is further known that in heavy duty tires used for trucks or buses it is advantageous to form a plurality of circumferential grooves extending along the tire circumferential direction and a plurality of lateral grooves extending in the tire width direction, to define a so-called block pattern, in order to obtain high drainage performance and so on. The present inventor is concerned with developing a tire having a tread pattern which combines the block pattern with the high angle uni-directional pattern for higher drainage performance.
In general, a tire having a normal block pattern has tendency to undergo uneven wear so-called heel-and-toe wear, in which a kick-out portion of the block largely wears when the tire is rotated under load. This tendency is remarkable in a heavy duty tire because it is used under conditions of high tire-inner pressure and contacting pressure.
It is considered that the main cause of such uneven wear is the compressing transformation by the input from the ground when the step-in portion of the block contacts with the ground. That is, compressing transformation is accumulated as energy in the block while the block is in contact with the ground, and such accumulated energy is released when the kick-out portion of the block is detached from the ground, whereby a slip is generated between the kick-out portion and the ground accompanied with the release of said accumulated energy. This is continuously repeated, and as a result the kick-out portion undergoes wear.
As a means for controlling such uneven wear, it is advantageous to enhance the rigidity of the step-in portion of the block, whereby there is controlled the compressing transformation by the input from the ground when the step-in portion of the block is contacted with the ground, and the energy accumulated in the block is lowered. As a result the slip between the kick-out portion and the ground is controlled, as described in Japanese Patent Application Laid-open No. Hei 5-270214.
However, it has been ascertained that in the case of a tire having a high angle uni-directional block pattern, the uneven wear generates not only at the kick-out portion but also at the step-in portion of the block, which is peculiar to the above pattern.
Therefore, an object of the present invention is to provide a pneumatic tire having a uni-directional block pattern, especially a high angle uni-directional block pattern, at the tread portion, which controls uneven wear which tends to generate both at the kick-out portion and the step-in portion of the block, and maintains high drainage performance, by adjustment of the rigidity of the side wall portions of the block.